The invention further relates to an apparatus for implementing the method.
Such a method and such an apparatus are known from U.S. Pat. No. 7,103,141 B2. According to the known method, a head support is used for determining exposure values in an X-ray cephalometric apparatus. The head support comprises support elements that can be positioned against the head of a patient, preferably against the cranial part of the head. The position of the support elements is determined by mechanical sensors. From the position of the support elements, the size and the position of the head of the patient can be derived. In particular, the distance between the ears of the patient and the position of the nasion can be determined. According to the data obtained a modulation of the radiation intensity or scanning speed of the X-ray exposure either automatically or according to a predefined profile is performed during the imaging process.
A similar method is disclosed in U.S. Pat. No. 6,510,196 B2. According to the known method various alternative size measurements of the head of a patient are used to estimate the bone thickness of the head. According to the data obtained for the size and position of the head, appropriate values of the current and voltage applied to the X-ray tube as well as appropriate values of the exposure time are proposed to the operator on a display. The operator may then accept the proposed values or change the actual settings of these operational parameters.
In the known methods, the settings of the operational parameters are chosen based on the aperture of the head support. This approach is highly affected by inaccuracy due to the differences in age, sex and height. Moreover, positioning the patient correctly inside the head support is a time consuming step, and the comfort of the patient is not optimized due to the head support.
U.S. Pat. No. 4,856,038 discloses a method for adjusting a panoramic X-ray apparatus. In the known method, the most appropriate dental arch profile for the actual patient is identified by a subjective evaluation of the operator supported by a tablet on which the actual patient dental arch profile can be drawn by the operator. This will be used by the system to drive the movements during the panoramic imaging and hence to adjust the layer in focus around the selected dental arch profile.
According to other known methods the operational parameters of the X-ray exposure are automatically controlled during the imaging process.
In the methods according to US 2004/0096035 A1, the radiation dose is increased or decreased by a constant factor by an automatically exposure control (=AEC) system, on the basis of the signal measured along the first acquired columns of a panoramic radiography. Other refined AEC systems, including closed-loop operation, have been proposed for digital mammography in ELBAKRI, I. et al.: Automatic Exposure control for a slot scanning full field digital mammography systems, Med. Phys. 32, p. 2763-2770, 2005.
Also panoramic X-ray equipments of the prior art, such as the one disclosed in U.S. Pat. No. 5,425,065, typically adopt a modulation of the X-ray exposure to compensate the higher absorption in the region of the spine. Such modulation can be either predefined or automatically adjusted by an automatic exposure control during the acquisition process. The result is often not optimal due to lack of adjustment on the specific anatomy and positioning of the actual patient, or due to inaccuracies of the automatic exposure control system. This may generally be corrected by post exposure image processing, aimed at uniformity of the image density along the various regions, but it does not compensate the lack of signal-to-noise ratio due to incorrect exposure, which typically exhibits in vertical bands on the diagnostic image.
Some effective software solutions for correcting a posteriori these defects have been proposed in FROSIO, I.; BORGHESE, N. A.: A New Real Time Filter for Local Exposure Correction in Panoramic Radiography, Medical Physics, Vol. 33, No. 9, September 2006, p. 3478-88. Another method is disclosed in FROSIO, I.; FERRIGNO, G.; BORGHESE, N. A., Enhancing digital cephalic radiography with mixture models and local gamma correction, IEEE Trans Med Imaging. Vol. 25 No. 1, January 2006, p. 113-121.
However, these algorithms cannot guarantee that the signal-to-noise ratio is constant over the entire image. Moreover, they locally modify the statistical properties of the image and this can represent a problem for further processing algorithms applied to these images. Best results could be achieved only by optimizing a priori the X-ray modulation.
U.S. Pat. No. 7,133,496 B2 discloses a method for cephalometric radiography in which an almost correct exposure of both the bone and the soft tissue is obtained by means of some hardware filter applied to the radiographic apparatus, aimed at the reduction of the dose in the soft tissue area.
Such procedures are usually complemented by specific image post processing, aiming to apply a differentiated contrast enhancement gamma for the regions of soft tissues and the regions of bone tissue, as described in EP 1 624 411 A2.
Besides these methods for adjusting the operational parameters of an X-ray apparatus, methods have recently been developed for automatic face recognition systems. An overview can be found in SINHA, P. et al. in: Face Recognition by Humans: 19 Results All Computer Vision Researchers Should Know About, Proceedings of the IEEE, Vol. 94, No. 11, November 2006, p. 1948-1962 and in ZHAO, W. et al. Face Recognition: A Literature Survey, ACM Computing Surveys, 2003, p. 399-458
The publications TISSE, C. et al., Person identification technique using human iris recognition (2002), IEEE Trans. Patt. Anal. Mach. Intell., 2002 and FOX, N. A. et al., Robust Biometric Person Identification Using Automatic Classifier Fusion of Speech, Mouth, and Face Experts, IEEE Trans. On Multimedia, Vol. 9, Issue 4, June 2007 p. 701-714. as well as FOX, N. A.; REILLY, R. B., Robust multi-modal person identification with tolerance of facial expression, 2004 contain further details on face identification.
SABER, E.; TEKALP, A. M., Frontal-view face detection and facial feature extraction using color, shape and symmetry based cost functions, Pattern Recognition Letters, Volume 19, Issue 8, June 1998, p. 669-680 describes a method for detecting human faces based on color and shape information and for locating the eyes, nose and mouth by symmetry based cost functions.
GOTO, T. et al., Facial feature extraction for quick 3D face modeling,” Signal Processing: Image Communication, Volume 17, Number 3, March 2002, p. 243-259(17) discloses a method for the three-dimensional face modeling.
A similar method can be found in BLANZ, V.; VETTER, T., Face Recognition Based on Fitting a 3D Morphable Model, IEEE Transactions on Pattern Analysis and Machine Intelligence, vol. 25 no. 9, p. 1063-1074, 2003 and also in BLANZ, V.; VETTER, T., A Morphable Model for the Synthesis of 3D Faces, SIGGRAPH 99 Conference Proceedings.
Vision systems were already proposed in radiotherapy to control the correct alignment of the patient with respect to the X-ray machine. According to JOHNSON, L. S. et al., Initial clinical experience with a video-based patient positioning system, International Journal of Radiation Oncology, Biology, Physics, Volume: 45, Issue: 1, Aug. 1, 1999, p. 205-213 a vision system is used for positioning a patient with respect to reference positions; the position of the patient can then be checked in each moment through a simple image subtraction technique.
A first dental X-ray system equipped with video cameras was disclosed in DE 36 32 878 A1, where the use of video cameras is proposed for the purpose of generating silhouettes of the head of a patient and adjusting the silhouette of an actual position with the silhouette of a desired position of the patient head.
Another dental X-ray system equipped with video cameras is disclosed in JP 2001 34 67 96 A, where the use of a video camera is proposed for verifying or controlling, either manually or automatically, the positioning of a head of a patient.
According to U.S. Pat. No. 6,614,875 B1 a cephalographic X-ray system is equipped with a video camera. In this document, the use of a video camera with the same geometry as the X-ray field is proposed for obtaining cephalographic pictures and side views of the head that can be superimposed by means of reference elements in the head positioning means.
A further dental X-ray system equipped with video cameras is disclosed in US 2007/0183567 A1, where the use of one or more video cameras is suggested for identifying a misalignment of the head with respect to the system by automatic processing and analyzing the video images, and applying a correction of the relevant mechanical parameters guided by reference markers or lines superimposed on the views of the patient.
Finally, WHITAKER, R. T, A Level-Set Approach to Image Blending, Image Processing, IEEE Transactions on, Volume 9, Issue 11, November 2000 pp 1849-1861 discloses a method for blending images.